it only helps a little bit because it only saves the need to store some of the transaction headers. As far as the information inside of the body is concerned, you still have just as many inputs and outputs, its just as much data to be stored on the blockchain.
It could help if it covered time too.
If A pays B and then B pays C, then the 2 transactions could be combined to give A pays C.
A pays C could have higher fee per kB than A pays B and B pays C, so it is worth it for a mining pool to combine them.
This could happen if the transaction pool started becoming a queue. At the moment, transactions mostly get into the chain.
If transactions stayed unconfirmed for a while, then you could end up with more opportunities to collapse the transactions. So, a longer queue makes the blockchain more efficient.